1. Technical Field
The present invention relates to a three-piece oil-control ring for use on the piston groove of an internal combustion engine, comprising a first upper/higher annular segment and a second lower annular segment, associated to an intermediate expanding element provided with a geometry that exhibits thickness, taken adjacent its internal end (smaller diameter) substantially smaller than the thickness taken in the region of association with the first and second annular segments.
The geometry of the expanding element prevents undesired contact with the inner wall and the upper/higher and lower edges (angles) defined by the piston groove, decreasing the tolerance accuracy required in the manufacturing process of this component. In this way, one achieves a reduction in the piston manufacture cost and a smaller number of incompatibilities between the expander and the groove, reducing to zero the occurrence of a situation called “pop-out”, corresponding to the distance between the inner diameter of the ring when it is free, that is, not confined in the cylinder, with respect to the walls of the outer diameters close to the groove flanks where the ring is housed. The larger the pop-out the greater the risk of occurring inadequate mounting by causing the rings to deform and lock at the moment of inserting piston and rings into the cylinder.
2. Related Art
Internal combustion engines are energy transforming mechanisms used by most automotive vehicles, and comprise basically two main parts: one or more cylinder head and the engine block. On the base of the cylinder head(s) are located the combustion chambers (in Diesel engines the combustion chambers are generally at the piston heads) and in the engine block are located the cylinders and the crankshaft assembly. The crankshaft assembly is composed by pistons, rods and the crankshaft.
The engine converts the energy produced by combustion of the mixture (fuel and air) in the combustion chambers to mechanical energy capable of driving the wheels.
Since the driving force required for moving the automobile comes from the burning of the air/fuel mixture in the combustion chamber, and in order to ensure a homogeneous combustion, without burning oil, and still prevent the excessive passage of gases from the cylinder to the oil pan, it is necessary to use rings to provide good sealing of the clearance existing between the piston and the cylinder wall.
In most present day internal combustion engines that operate according to the Otto and Diesel cycles three rings are used, two of which being compression rings and one being oil control ring (scraper). The compression rings have the function of preventing the passage of gases from the combustion into the oil pan and the oil ring has the function of scraping the excess of oil from the cylinder wall and return it to the oil pan, controlling the thickness of the oil film, so that the operation of the engine will take place within the design and operation premises.
Another important function of the rings is serving as a bridge for transmitting heat from the piston to the cylinder wall/jacket, where heat dispersion occurs through the cooling system.
The oil scraping rings may be of one, two or three pieces. The present solution has been developed in the form of a three-piece ring that, as a rule, has first upper/higher annular segment and second lower annular segment, associated to an intermediate expanding element, which presses the upper/higher and lower segments against the cylinder wall in a controlled manner. The force exerted by the expanding element, which is nothing more than a resilient element, is calculated so that the oil film on the cylinder wall will have the desired thickness.
Although the 3-piece oil scraping rings are quite specified by the engine designers, there are a few disadvantageous characteristics coming from the constitution thereof, the greatest of which being the need to design the expanding element so that it will never interfere with the inner piston wall and the upper/higher and lower edges (angles) defined by the piston groove (the third groove, farther away from the groove).
This is achieved, in most cases, by applying a more reduced tolerance variation, namely 0.3 millimeter in the dimensions of the upper/higher and lower groove borders. Starting from this low dimensional variation, the expanding element is designed so that its proportions, under consideration of the tolerances in manufacturing it, will not cause undesirable interference with the groove in its most critical dimension, in view of the tolerance variation. In other words, groove and expander should be sized so that, even in the most critical situation of interference, no interference will occur.
Due to this kind of sizing, an opposite situation may occur, when the dimensions of groove and expander are such that the opposite situation of maximum clearance between them will occur, which may lead to the undesirable phenomenon of “pop-out”, mentioned above.
There are various prior techniques referring to three-piece oil scraping rings, proposing improvements on this constructive concept, some of which are briefly listed hereinafter.
Document U.S. Pat. No. 7,854,191 relates to a three-piece oil scraping ring, designed to reduce the consumption of lubricating oil and to increase the sealing properties. The expanding element has support portions (resting protrusions) to enable positioning and fixing of the two annular segments. In order to enable the ring to perform as desired, the resting protrusions exhibit an angle between 10 and 20 degrees with the vertical and a geometric relationship 2X/Y between 0.04 and 0.15, wherein 2X corresponds to the projecting length of a respective support protrusion of the segment in the axial direction and Y is the distance between the distal end faces of the protrusions in the axial direction.
Document US 2006/0061043 relates to a 3-piece oil scraping ring, the expander of which has protrusions (which are in contact with the annular segments) made from austenitic stainless steel and coated with a nitrided coating layer.
Finally, document U.S. Pat. No. 4,798,391 relates to a 3-piece oil scraping oil, the expander of which exhibits slots and ribs that contribute to obtaining a desired rigidity value, reducing the consumption of lubricating oil of the engine equipped therewith.
The three documents cited above show different approaches to the improvement of three-piece oil scraping rings, with a view to increase its performance, which has the result of reducing the consumption of lubricating oil. But there is no concern, in any of these documents, about the shape of the cross section of the expander with regard to the dimension tolerance of the groove and the occurrence of the pop-out phenomenon, which undoubtedly bring drawbacks while assembling the engine.
Until the present moment, no three-piece oil scraping ring had been developed with an expander designed geometrically to prevent the need for great tolerance control of the piston groove, while eliminating the occurrence of the pop-out phenomenon.